That’s my tentative estimate (based on Google election result and population statistics) of the percentage of Americans who voted for nobody for President of the United States on Tuesday.

US President Barack Obama knocked down about 60.7 million votes.

GOP challenger Mitt Romney polled about 57.8 million.

Libertarian nominee Gary Johnson, 1.14 million.

Green candidate Jill Stein, about 400,000.

A few others, a few thousands or tens of thousands.

About 38.8% of the population supported one of the candidates; about 19.5% of the population supported the alleged “winner.”

61.2% of the population did not consent to be ruled at all, and fewer than one in five Americans consented to be ruled by Barack Obama. The figures are likely similar for most or all of the 435 US Representatives and 33 US Senators “elected” on Tuesday.

If these politicians support the system of government they claim to support — one in which governments “deriv[e] their just powers from the consent of the governed” — then the only order of business they have to discuss is who will turn the lights off as they depart Washington.

However, not all of that 61.2% abstained by choice. Approximately 25% of the US population is under age 18, and so not eligible to vote. (We can argue about the fairness of this another time, for now my point is simply that, as of Tuesday, they didn't have that option.) Another 1.7% of the population is denied the right to vote because of felony disenfranchisement. Certainly none of these ineligible voters consented to be ruled, but I think it's worth distinguishing between those who chose not to vote and those who weren't given a choice.

So, of the potential voter pool (not registered, but those who could be registered), roughly 228 million, about 26.7 supported Obama, about 25.4% supported Romney. About 46.9% of eligible voters abstained.

I would contend that a person's legal eligibility as a voter isn't relevant to the point that Knapp was trying to make. Instead of the size of the voter pool, we should concern ourselves with the size of the governed pool, everyone who finds themselves under the authority of the offices constituting the United States of America. With this expanded but more significant pool in mind, the legitimacy of these offices becomes even more tenuous.

My estimate included just adults, but didn't cut out felony disenfranchisement. So it's somewhere in between what Tom has here, and what you recommended, but my numbers came to about 47% for Nobody.
My recent post 2012 USA Presidential Election Results